Precision Predictions for Flavour-Changing Neutral Current Decays

由变味中性流(FCNCs)引起的夸克衰变是探索粒子物理标准模型(SM)边界的重要工具。在SM中，这些衰变被Cabibbo-Kobayashi-Maskawa(CKM)矩阵中的小混合角和/或轻夸克的小Yukawa耦合(GIM机制)所抑制。在SM的扩展中，类似的有效抑制机制并不一定实现，因此“新物理”(NP)信号可能以衰减率和不对称性的形式出现。在理论方面，用有效哈密顿量描述了低能FCnC过程，有效哈密顿量允许考虑SM及其可能的扩展的贡献，例如，包括来自右手电流或CP破坏的新来源的影响。到目前为止，我们已经观察到了许多基于b夸克到S夸克跃迁的过程；第一批数据也提供了b到d衰变模的第一个数据，这些衰变模被更小的CKM角进一步抑制。涉及上型夸克(u，c，t)的FCNCs的情况不那么先进，部分原因是对相应衰变速率的抑制更强。在这个项目中，我们计划对含有b夸克的强子的各种独有的FCnc衰变进行全面的理论研究。这种衰变非常适合于测试SM内外的基本相互作用，因为一方面，未来几年LHC实验有望获得大量新数据，同时，由于重b夸克质量的逆幂的系统展开，这些模式中的强子不确定性得到了合理的理论控制。通过考虑到对强子参数的改进的理论预测，例如跃迁形状因子，以及通过构造对强子不确定性的灵敏度降低的优化的观测值，我们将探索和分类角分布和衰变不对称中可能的NP效应的各种特征。此外，我们还计划用类似的理论方法研究c夸克的稀有Fcnc衰变。

Quark decays which are induced by flavour-changing neutral currents (FCNCs) represent important tools to explore the borders of the Standard Model (SM) of particle physics. In the SM, these decays are suppressed by small mixing angles in the Cabibbo-Kobayashi-Maskawa (CKM) matrix and/or small Yukawa couplings of light quarks (GIM mechanism). In extensions of the SM, similar effective suppression mechanism are not necessarily realized, and therefore “new physics” (NP) signals can show up in decay rates and asymmetries. On the theoretical side, FCNC-processes at low energies are described in terms of effective Hamiltonian which allows to take into account contributions from the SM as well as from its possible extensions, including, for instance, effects from right-handed currents or new sources of CP-violation. To date, we have already observed a multitude of processes based on b-quark transitions to s-quark; first data have also become available for b to d decay modes, which are further suppressed by smaller CKM angles. The situation for FCNCs involving up-type quarks (u, c, t) is less advanced, partly because of the still stronger suppression of the corresponding decay rates. Within this project, we plan to perform a comprehensive theoretical study of various exclusive FCNC decays of hadrons containing b-quarks. Such decays are ideally suited to test the fundamental interactions in and beyond the SM because, on the one hand, lots of new data can be expected from the LHC experiments in the coming years, and, at the same time the hadronic uncertainties in these modes are under reasonable theoretical control thanks to the systematic expansion in inverse powers of the heavy b-quark mass. By taking into account improved theoretical predictions on hadronic parameters, such as transition form factors, and by constructing optimized observables with reduced sensitivity on hadronic uncertainties, we are going to explore and classify various signatures of possible NP effects in angular distributions and decay asymmetries. In addition, we plan to investigate rare FCNC decays of c-quarks with similar theoretical methods.